U.S. patent number 7,896,986 [Application Number 10/932,718] was granted by the patent office on 2011-03-01 for heat treatment of superalloy components.
This patent grant is currently assigned to Siemens Energy, Inc.. Invention is credited to Peter J. Ditzel, Srikanth C. Kottilingam.
United States Patent |
7,896,986 |
Kottilingam , et
al. |
March 1, 2011 |
Heat treatment of superalloy components
Abstract
An improved method of heat treating superalloys prior to welding
includes subjecting only the portion of the component to be
repaired to a localized heat treatment, leaving the remainder of
the component untreated. The localized heat treatment permits the
use of higher hold temperatures that are near, at, or above the
Ni.sub.3(Al,Ti) solution temperature of the alloy. Such heat
treatment prevents strain age cracking and also prevents
recrystallization in areas that are not heat treated. Such
localized heat treatment can be applied before and/or after
welding, for material rejuvenation, pre-brazing, and
post-brazing.
Inventors: |
Kottilingam; Srikanth C.
(Orlando, FL), Ditzel; Peter J. (Orlando, FL) |
Assignee: |
Siemens Energy, Inc. (Orlando,
FL)
|
Family
ID: |
35941372 |
Appl.
No.: |
10/932,718 |
Filed: |
September 2, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060042729 A1 |
Mar 2, 2006 |
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Current U.S.
Class: |
148/675;
29/889.7; 148/562; 228/119 |
Current CPC
Class: |
C22F
1/10 (20130101); Y10T 29/49336 (20150115) |
Current International
Class: |
C22F
1/10 (20060101) |
Field of
Search: |
;148/672,675 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
David A Deantonio et al., "Heat Treating of Superalloys" ASM
Handbook, vol. 4, pp. 793-814, 1995. cited by examiner.
|
Primary Examiner: King; Roy
Assistant Examiner: Kessler; Christopher
Claims
The invention claimed is:
1. A method of locally repairing a Ni.sub.3(Al,Ti) superalloy
component, the method comprising: locally heating, in a heat zone,
only the portion of the component that requires repair at a
temperature close to, at, or above the Ni.sub.3(Al,Ti) solution
temperature with a hold temperature of the portion of the
superalloy component that requires repair in the range of
1,850.degree. F. to 2,400.degree. F., with a sufficiently high hold
temperature to prevent strainage cracking; while the remainder of
the component, not being in the heat zone, does not undergo any
separate heating and retains its original microstructure devoid of
any recrystallization; performing a repair, selected from the group
consisting of welding and brazing, upon the locally heated portion
of the superalloy component that requires repair; and then cooling
the locally heated portion of the component that requires repair
from the hold temperature at two rates over a time period totaling
three to ten hours, to prevent diffusion of molecules and to resist
recrystallization in locations where repair is not necessary, and
to resist cracking in the heat affected zone, said cooling, at a
first rate, where upon reaching a cooling temperature of about
1,200.degree. F. to 1,700.degree. F., the portion of the component
that requires repair is cooled at a second rate more rapidly than
at the first rate; where at the slower first rate continued
diffusion of molecules is permitted while at the more rapid second
rate further diffusion of the molecules is limited, wherein the
superalloy component is an equiaxed material, a directionally
solidified material, or a single crystal material, cooling is by a
gas cooling medium directed immediately adjacent the heat zone, to
carry away heat, and the superalloy component is a blade of a
combustion turbine.
2. The method according to claim 1, wherein the cooling is by a gas
medium and the medium is argon gas.
3. The method according to claim 1, wherein the localized heating
of the portion of the-component that requires repair is performed
in air.
4. The method according to claim 1, wherein the localized heating
of the portion of the component that requires repair is performed
in a chamber that is filled with inert gas.
5. The method according to claim 1, wherein the localized heating
of the portion of the component that requires repair is performed
in a vacuum.
6. The method of claim 1, wherein the locally heated portion of the
superalloy component that requires repair is cooled from the hold
temperature at a first rate between about 0.5.degree. F./min. to
about 5.degree. F./min., to a second rate to result in temperatures
in the range of about below 1,000.degree. F.
7. The method of claim 1, wherein the superalloy component is the
tip of an airfoil of a combustion turbine blade, and wherein only a
localized portion of the entire component is heat treated, so that
the portion of the component to be repaired is given a heat
treatment at a sufficiently high hold temperature to necessitate
the required averaging heat treatment to prevent strain age
cracking, while the remainder of the component does not undergo any
heat treatment and therefore retains its original microstructure,
this being effective to improve the repairability of the superalloy
component.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of repairing superalloy
components. More specifically, the invention provides a method of
local heat treatment of superalloy components prior to welding in a
manner that resists recrystallization of the material in locations
where repair is not necessary, and also resists cracking in the
heat affected zone of the weld and deposited weld metal while
preserving the material properties of the remainder of the
component.
2. Description of the Related Art
Components of various types of equipment that are subject to high
temperature, high stress environments, for example, components
within combustion turbines, are typically made from materials known
as superalloys, which are defined herein as nickel based alloys
containing aluminum and/or titanium, or cobalt based alloys.
Components made from these materials typically include equiaxed
materials, directionally solidified materials, or single crystal
materials. After casting, the components are typically subjected to
various heat treatments, for example, homogenization, hot isostatic
pressing, solutionizing, and/or aging. The heating rate, hold
temperature, hold time, and cooling rate of these heat treatment
processes are intended to produce optimally sized and shaped grains
of precipitate of Ni.sub.3(Al,Ti) and carbides within the material.
The volume percentage, size, and distribution of these
precipitates, along with the type and distribution of the carbide,
determine the mechanical properties of the material. An optimum
volume percentage and distribution of precipitates is the source of
the material's high temperature strength.
During the operation of a combustion turbine having such components
therein, the high temperature and stress to which the components
are subjected cause precipitation of carbides in the grain
boundaries in equiaxed and directionally solidified materials, and
also causes coarsening of the Ni.sub.3(Al,ti) precipitates, thereby
changing the mechanical properties of the material. Prolonged
exposure to such conditions may cause cracking within the
material.
Such cracks are typically repaired by welding, however, superalloys
are difficult to weld. During welding, hot cracking may occur in
the heat affected zone due to liquation of low melting phases such
as borides, carbides, sulfides and/or phosphides in the grain
boundaries. Present efforts to reduce hot cracking include design
of the weldments, controlling trace elements within the base metal,
using lower strength weld filler metals, and using welding
processes with low heat inputs.
Additionally, post weld heat treatment cracking, also known as
"strain age cracking," may occur during the post weld heat
treatment which is performed to restore the properties of the
components and to relieve residual stresses within the material.
Such cracks may extend beyond the heat affected zone through the
weld metal or through the parent material. During heat treatment,
as residual stress is relaxed, precipitation of Ni.sub.3(Al,Ti)
occurs rapidly, resulting in volume contraction and strengthening
of the material, thereby resulting in a reduction of the ductility
of the material. Cracking occurs when the strain associated with
stress relaxation exceeds the strain capacity of the heat affected
zone. Hot cracks may act as the initiation points for strain age
cracks.
The strain-age cracking tendency of superalloys is related to the
total amount of alloying elements such as Al and Ti contained
within the alloy.
Presently used methods to minimize strain age cracking include
solution and overaging pre weld heat treatments. The former method
works well with alloys with low Ni.sub.3(Al,Ti) volume percents,
while the latter method works best for materials with high
Ni.sub.3(Al,Ti) volume percent. Such heat treatment typically
involves heating the entire component in a vacuum furnace to a
predetermined temperature and cooling the component to room
temperature, with the cooling done quickly or slowly depending on
the desired result. A typical hold temperature is the solution
temperature where all the Ni.sub.3(Al,Ti) precipitates go into
solution.
In the case of directionally solidified or single crystal
materials, the heat treatment hold temperature is limited to
temperatures that are lower than the solution temperature due to
recrystallization (formation of new small grains) within the
material. Formation of recrystallized grains results in a reduction
of the desired mechanical properties of the material. However, such
low temperature heat treatment is insufficient to improve the
weldability of the material.
Accordingly, there is a need for a method of heat treating
superalloys in a manner that improves the weldability of the
portion of the component to be repaired without damaging the
microstructure and material properties of the remainder of the
superalloy component. Such a method would substantially reduce the
cost of maintaining equipment using superalloy components by
improving the repairability of these components, and avoiding the
expense of replacement of damaged components.
SUMMARY OF THE INVENTION
The present invention provides an improved method of heat treating
superalloy components. The method includes performing a local
pre-weld heat treatment only to the region of the component that
requires repair. By using a localized heat treatment, temperatures
close to, equal to, or greater than the Ni.sub.3(Al,Ti) solution
temperature may be used. Such localized heat treatment will resist
recrystallization in other critical areas such as, in the example
of a turbine blade, the remainder of the airfoil and the root.
During localized heat treatment, the heat treated portion of the
component will be taken to a temperature between about
1,850.degree. F. and 2,400.degree. F. This portion of the component
may be allowed to cool from this temperature to approximately
1,000.degree. F. and 1,800.degree. F. at a controlled cooling rate.
The remainder of the component will generally be kept below
1000.degree. F. to resist alteration of the microstructure. Heat
conduction through the superalloys that is being given a localized
heat treatment is unlikely to be sufficient to increase the
temperature of the remainder of the component above about
1,000.degree. F. However, as an additional precaution, a cooling
medium may be directed below the portion of the component being
given a heat treatment, for example, directing Argon gas below the
heat treated portion to carry away the heat.
The region of the components in which welding will be performed may
be heat treated using well known local heat treating methods such
as induction heating or resistance heating. Particular superalloys
with which the present invention may be used include, but are not
limited to, CM247, MarM002, IN738, and RENE 80.
Accordingly, it is an object of the present invention to provide a
method of resisting cracking during weld repairs of components made
from superalloy materials.
It is another object of the invention to provide a method of
localized heat treatment of superalloy components.
It is a further object of the invention to maintain the
microstructure and mechanical properties of the portion of a
superalloy component outside the heat affected zone of a weld
repair.
It is another object of the invention to maximize the lifespan and
improve the repairability of components made from superalloys that
are used in high-temperature, high-stress environments.
These and other objects of the invention will become more apparent
through the following description and drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph illustrating the weldability of various alloys
based on susceptibility to strain age cracking.
FIG. 2 is a graph indicating the typical thermal cycle involved
during welding and post weld heat treatment.
FIG. 3 is a scanning electron microscope image magnified 10,000
times illustrating the Ni.sub.3(Al,Ti) precipitate size resulting
from pre-weld heat treatment with a high hold temperature.
FIG. 4 is a scanning electron microscope image magnified 10,000
times showing the Ni.sub.3(Al,Ti) precipitate size resulting from
pre-weld heat treatment with a low hold temperature.
FIG. 5 is a graph illustrating the difference in temperature and
time between prior whole component heat treatment and the localized
heat treatment of the present invention.
FIG. 6 is an isometric view of a blade for a combustion
turbine.
FIG. 7 is a metallograph magnified 100 times illustrating
recrystallization in a directionally solidified nickel base
alloy.
Like reference characters denote like elements throughout the
drawings.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention provides an improved method of heat treating
superalloys, which resists the formation of recrystallized grains
in portions of the component not being repaired, and also resists
cracking during and after the welding process. Although primarily
intended for use prior to welding, the heat treating method may
also be advantageously used after welding, to rejuvenate components
after extended service, and as a pre-brazing or post-brazing heat
treatment.
Referring to FIG. 1, the difficulty in welding various superalloys
based on their aluminum and titanium concentrations is illustrated.
As shown in FIG. 1, increasing concentrations of both aluminum
and/or titanium in nickel based superalloys increases the
difficulty of welding these materials. The graph shows that the
alloys CM247, MarM002, IN738, and RENE 80 are particularly
difficult to weld. All of these alloys are examples of alloys with
which the present invention may be used.
Referring to FIG. 2, the thermal cycle during welding and post heat
treatment is illustrated. Welding subjects the material to a very
high temperature for a relatively short period of time, resulting
in residual stress within the material. During the post-weld heat
treatment, two competing processes occur simultaneously. The
desired relief of residual stresses and the undesired
Ni.sub.3(Al,Ti) precipitation occur at the same time. Carbides may
also precipitate out of the material. FIG. 3 illustrates the
enlarged grain structure that results from high temperature heat
treatment of superalloys. This grain structure is contrasted with
FIG. 4, illustrating the small grain size that is desired for these
superalloys. This precipitation of Ni.sub.3(Al,Ti) particles
increases the strength of the material, with a concurrent reduction
in ductility. The combination of the relief of residual stresses
and precipitation of Ni.sub.3(Al,Ti) particles results in strain
age cracks. The present invention therefore seeks to avoid the
formation of recrystallized grains as shown in FIG. 6 in other
areas of the component for which repair is not needed, while
simultaneously preventing strain age cracking in the heat affected
zone and filler metal of the weld.
FIG. 5 illustrates the pre-weld heat treating process of the
present invention as compared with the previous heat treating
method. Both the previous method and the present invention utilize
similar heating rates, as indicated by the portion 10 of the graph.
However, a previous method of heat treating the entire component
used a hold temperature below the Ni.sub.3(Al,Ti) solution
temperature, as indicated by graph segment 12, while the present
heat treating method uses a heat treating temperature close to, at,
or above the Ni.sub.3(Al,Ti) solution temperature, with the hold
temperature preferably in the range of about 1,850.degree. F. to
about 2,400.degree. F., as indicated by graph segment 14. The hold
time at the desired temperature for the present invention may be
approximately equal to or, if desired, longer than the hold time of
the previous heat treating method, also illustrated by the line
segments 12, 14. After the appropriate hold time, the previous heat
treating method cools the entire component to below 1,000.degree.
F. over a short time period that may be about two hours,
represented by the line segment 16. The present invention cools the
component over a time period totaling about three to ten hours,
represented by the combination of the line segments 18 and 20.
During the initial phase of cooling, cooling is allowed to proceed
slowly from the hold temperature, as indicated by the line segment
18. At the point 22, where the temperature of the heat treated
material is about 1,200.degree. F. to about 1,700.degree. F., the
component is cooled more rapidly, as indicated by the line segment
20. As is well known in the art of heat treating, the slow cooling
rate represented by the line segment 18 permits for continued
diffusion of the molecules within the material, while the faster
cooling rate illustrated by the line segment 20 limits further
diffusion of the molecules.
The region of the components in which welding will be performed may
be heat treated using well known local heat treating methods such
as induction heating, resistance heating, lamp heating, or other
known heating methods. Basically, induction heating utilizes a
copper coil with a power supply to induce eddy currents in the
component, with the eddy currents generating heat. Resistance heat
treatment utilizes resistance elements on or near the component
being heat treated. The heat treatment may be performed in air, in
an inert gas environment, or in a vacuum.
During the localized heat treatment, heat conduction through the
remainder of the superalloy component is unlikely to be sufficient
to raise the temperature of the remainder of the component above
1,000.degree. F. However, a cooling medium may be directed
immediately adjacent to the heat affected zone of the component
being repaired, for example, directing argon gas adjacent to the
heat affected zone to carry away the heat.
FIG. 6 illustrates a blade 24 of a combustion turbine, which is
representative of a component that may be repaired using the
present invention. The blade 24 includes a root depending downward
from a platform 28. An airfoil 30 extends upward from the platform
28. When the blade 24 is installed within a turbine, the root 26
will be retained by the turbine discs using a fir-tree
configuration and a locking mechanism. The tip 32 of the airfoil 30
will undergo the greatest stress during use, and is therefore the
most likely location for crack formation. In such a case, after
welding, only the heat affected zone 34 requires a heat treatment
of the present invention, thereby preserving the metallurgy of the
remainder of the blade 24. If desired, a coolant such as argon gas
may be applied to the region 36 of the airfoil 30.
The present invention therefore provides an improved method of heat
treating a superalloy component, wherein only a localized portion
of the entire component is heat treated. The portion of the
component to be repaired may therefore be given a heat treatment at
a sufficiently high hold temperature to necessitate the required
averaging heat treatment to prevent strain age cracking, while the
remainder of the component does not undergo any heat treatment and
therefore retains its original microstructure, devoid of any
recrystallization. The present invention therefore improves the
repairability of superalloy components used in high-temperature,
high-stress environments such as the inside of a combustion
turbine, thereby increasing the lifespan of these components and
decreasing the cost of maintaining a combustion turbine or other
equipment utilizing such superalloy components. The heat treatment
may be used pre-welding, post-welding, pre-brazing, post-brazing,
or for component rejuvenation. The heat treatment may be used with
equiaxed materials, directionally solidified materials, or single
crystal materials.
While a specific embodiment of the invention has been described in
detail, it will be appreciated by those skilled in the art that
various modifications and alternatives to those details could be
developed in light of the overall teachings of the disclosure.
Accordingly, the particular arrangements disclosed are meant to be
illustrative only and not limiting as to the scope of the invention
which is to be given the full breadth of the appended claims and
any and all equivalents thereof.
* * * * *